1. Field of the Invention
The present disclosure relates to a nanocrystal, a method for preparing the nanocrystal, and an electronic device comprising the nanocrystal. More specifically, the present disclosure relates to a nanocrystal comprising a semiconductor nanocrystal core, a non-semiconductor buffer layer surrounding the nanocrystal core, and a shell layer surrounding the buffer layer; a method for preparing the nanocrystal; and an electronic device comprising the nanocrystal. The nanocrystal described herein has a minimal number of surface defects, an improved luminescence capability, and an enhanced level of color purity.
2. Description of the Related Art
A nanocrystal is a single crystal particle having a cross-section of only a few nanometers, and consisting of several hundred to several thousand atoms. Since such a small-sized material has a large surface area per unit volume, most of the constituent atoms of the nanocrystal are present on the surface of the nanocrystal. As a result of this characteristic structure, a semiconductor nanocrystal is under the influence of quantum confinement effects, and thereby demonstrates electrical, magnetic, optical, chemical and mechanical properties that are substantially different from those inherent to the constituent atoms of the nanocrystal.
In other words, controlling the physical size and composition of the semiconductor nanocrystals enables control over the properties of the nanocrystals. Accordingly, semiconductor nanocrystals can be utilized in a variety of applications including: luminescent devices, for example, light-emitting diodes (LEDs), electroluminescence (EL) devices, lasers, holographic devices and sensors; and electronic devices, for example, solar cells and photodetectors.
Nanocrystals are generally prepared using wet chemistry methods wherein a precursor material is added to a coordinating organic solvent in order to grow the nanocrystals to an intended size. As the nanocrystals grow, the organic solvent is naturally coordinated to the surface of the nanocrystals, thereby acting as a dispersant. As such, the organic solvent facilitates the growth of the nanocrystals to a nanometer-scale level. The wet chemistry method has advantages in that nanocrystals of a variety of sizes can be uniformly prepared by appropriately controlling the concentration of the precursor materials used, the type of organic solvent used, as well as the preparation temperature and time.
A wide variety of different kinds of nanocrystals are known to date, and in general, Group II and Group VI compound semiconductors are the compounds most often used as nanocrystals. The Group II and VI compound semiconductor nanocrystals are easy to prepare, and they exhibit the desired optical properties, however they are problematic in that they contain heavy metals that fall under government environmental regulations. For these reasons, Group III and Group V semiconductor nanocrystals have been attracting a great deal of attention as potential replacements for the Group II and Group VI compound semiconductors.
Various methods to prepare nanocrystals, which are free of heavy metals, such as cadmium, mercury and lead, have been reported (The Handbook of Nano-structured Materials and Nanotechnology, Chapter 8; Journal of Luminescence 70 (1996) 70: 95-107; and U.S. Pat. No. 5,505,928). In addition, methods for synthesizing a core-shell nanocrystal structure are also known, wherein a crystal is grown on the surface of nanocrystals, and the crystal acts as a passivating shell in order to control the development of surface defects or, to employ quantum confinement effects with the purpose of improving the luminescence efficiency of the nanocrystals.
However, it is difficult to synthesize nanocrystals. In particular, the use of a wet chemical process, results in a precursor that has deteriorated reactivity, thereby making it difficult to grow the nanocrystals. Once synthesized, the nanocrystals are very poor in quality. Due to the small size of the nanocrystals, the presence of defects on the surface of the nanocrystals, results in the deterioration of the luminescence efficiency of the nanocrystals. In addition, it is also difficult to passivate a nanocrystal core with a shell layer. For this reason, there are technical limitations associated with the preparation of nanocrystals of high quality and superior luminescence efficiency. The reason for the difficulty in passivation is that a core-shell structured nanocrystal may involve non-uniform growth of the shell layer due to lattice mismatch between nanocrystals constituting the core and nanocrystals constituting the shell. Consequently, the interface between the core and the shell becomes unstable, thus disadvantageously causing deterioration in the luminescence and optical properties of the nanocrystal.